Thanks. I was experiencing problems with messages claiming inadequate memory, etc, so I decided to revert to a less troublesome desktop earlier in Sept. In the process, I lost some files that I had added after the date I entered. I guess I'm getting this process confused with Windows Restore, which won't lose recent files.
Anyway, a tech at the Apple store told me there should be a way to get these recent files, but he wasn't sure how to do it.
Any suggestions?

Time Machine is different from Windows Restore in that you don't use it to make restore points on your actual hard drive itself. You need to use an external hard drive, which makes sense because if your HD is damaged, there go all your restore points along with it.

TM will make a backup of your entire system every hour unless you disconnect the external drive. If you need to you can navigate to any previous date to restore any file, folder or application you might be missing. Here's a neat tutorial that should give you an idea of how it works.

Here's the problem -
On 9/30, in an attempt to get rid of some of the problems I mentioned above, I used Time Machine to take the entire Desktop to 9/8. Unfortunately, the file changes I made were AFTER 9/8, so they're gone. I'd like to know if there's a way to find the files for a date around 9/20 or 25, even though TM doesn't show anything between 9/8 and 9/30.